So here's the ground cooling off by radiating infrared radiation to space. The air in contact with the ground cools off by conduction, but cannot convect upward because the cold air is denser than the warmer air above it. One can see an inverse temperature profile develop in this shallow layer of cold air (i.e. temperature increases with increasing altitude instead of decreasing as it usually does in the troposphere). This is a temperature inversion, and because it is formed at night by the radiation of IR to space, it is also known as a nocturnal inversion or a radiation inversion.